This invention and the specific rationales for identifying and functionalizing taste specific genes were developed with their initial objective being the identification and functionalization of a gene encoding a salty taste receptor. With respect thereto, epithelial sodium channels (ENaC) are members of the ENaC/degenerin family of ion channels that includes acid-sensing ion channels (ASIC) in mammals, mechanosensitive degenerin channels in worms, and FMRF-amide peptide-gated channels in mollusks (Kellenger, S. and Schild, L. (2002) Physiol. Rev. 82:735-767). ENaC mediates amiloride-sensitive apical membrane Na+ transport across high resistance epithelia in numerous tissues including kidney, colon, and lung and have been well studied and predicted to be involved in salty taste in primates and other species.
ENaC is known to be a heterotrimeric channel comprised of alpha, beta, and gamma subunits or delta, beta, and gamma subunits. Particularly, this heterotrimeric channel has been hypothesized to be involved in human salty taste perception. Previously, assays have been developed by the present assignee using ENaC sequences to identify compounds that modulate the delta beta gamma and alpha beta gamma human ENaC to examine if these compounds will potentially modulate human salty taste perception. Also, these compounds potentially may be used to treat human pathologies involving aberrant ENaC function.
Unlike other mammals, amiloride only slightly reduces the intensity of sodium chloride taste, i.e., by about 15-20% when used at concentrations that specifically modulate ENaC function (Halpern, B. P. (1998) Neuroscience and Behavioral Reviews. 23: 5-47). Experiments conducted by the inventors have shown that amiloride, or the more potent amiloride derivative phenamil did not elicit a significant effect on perceived human salt intensity when tested at levels 300-fold (for amiloride) and 3000-fold (for benzamil) above IC50 values for alpha beta gamma ENaC (equivalent to 10-fold for amiloride and 100-fold for benzamil over IC50 values for delta beta gamma ENaC). Thus, additional non-ENaC genes are likely involved in human salty taste.
In addition, it has been recently reported that taste receptors may be expressed in non-oral tissues, e.g., in the digestive system and potentially other organs such as the kidney. Particularly it has been reported that sweet, umami and bitter taste receptors are expressed in cells other than in the oral cavity such as gastrointestinal cells. (See, e.g., Sternini et al., Amer J Physiol. Gastrointestinal and Liver Physiology, 292:G457-G461, 2007; Mace, O. J. et al, J. Physiology. 10.1113/jphysiol.2007.130906. Published online May 10, 2007). Also, it has been reported by various groups (Margolskee et al., Bezencon et al., Rozengurt et al, and Sternini et al. (2007) (Id)) that bitter and umami taste receptors and other taste signaling molecules such as TRPM5 and gustducin are expressed in specialized cells in the gastrointestinal tract. (See e.g., Margolskee et al., Genes Brain Behavior 2007 (epub March 21); Rozengurt et al., Amer. J. Physiol. Gastroent. Liver Physiol. 291(2):G171-7 (2006); Bezencon et al., Chem Senses 32(1):41-47 (2007)). Margolskee et al. (Id) further reports that the loss of T1R3 or gustducin in rodents resulted in changes in insulin metabolism and the release of satiety peptides such as GLP-1 (glucagon-like peptide 1).
Based on the foregoing, it has been suggested that salty receptors may be expressed in the urinary tract. Taste receptors are purported to be involved in functions not directly related to taste such as digestive functions such as gastric motility, absorption, food detection, metabolism, and immune regulation of the oral or digestive tract and may also affect functions relating to sodium absorption, excretion and transport such as blood pressure and fluid retention.
Therefore, the identification of taste cell specific genes and identifying what specific cells these genes are specifically expressed (including unique taste cell subsets) should facilitate a better understanding of taste and non-taste functions of these taste receptors and should also facilitate the use of these genes, gene products and cells which express same in assays for identifying novel taste modulators and therapeutics, e.g., for treating digestive diseases such as autoimmune, inflammatory and cancers, metabolism, diabetes, eating disorders, obesity, taste cell turnover, hypertension, fluid retention, and immune regulation of the digestive system.